The coupled effect of surface roughness and nanoparticle size on the heat transfer enhancement of nanofluids for pool boiling

Abstract

In the present work, the combined effect of surface roughness and nanoparticle size, also known as surface-particle interaction parameter (SPIP) and defined as the ratio of surface roughness to the particle size, was investigated numerically by simulating nanofluid/vapour two-phase pool boiling inside an unsteady 2-D symmetric chamber consisting of a heat sink as the heated wall. To account for the SPIP, new correlations for bubble departure diameter and nucleation site density were implemented as a user-defined function in ANSYS Fluent. The bubble waiting time coefficient was corrected at different nucleation site density during validation study where good agreement was found and then the same bubble waiting time coefficients were used during the rest of the investigations accordingly. The effect of nanoparticle concentration, fin aspect ratio, number of fins and different base fluids were also investigated. Aluminium oxide was used as the nanoparticle throughout this study. The results showed that when the SPIP is near 1, the lowest heat flux is achieved and thus will always show an inferior performance in heat transfer when compared to pure water. As SPIP increases past 1, higher heat transfer coefficient and heat flux is achieved and thus will show an enhancement in heat transfer performance when compared to water at appropriate concentrations. When SPIP is lower than 1, the heat flux is lower than when SPIP is higher than 1 but still higher than when SPIP is near 1. It was also found that as the number of fins and fin aspect ratio increases, the heat transfer coefficient increases. There is, however, a deterioration in heat transfer when the nanoparticle concentration increases. It was found that at SPIP close to 1, water based nanofluid always shows far better heat transfer capabilities than refrigerant based nanofluids. However, at SPIP: 16, R245FA based nanofluid achieves higher heat flux than water based nanofluid at higher wall superheat temperatures.